1. Field
The present invention relates to printed electronics and, in particular, the creation of circuit elements using copper nanoparticles.
2. Description of the Related Art
Electrical assemblies are currently fabricated using a rigid substrate with individual components attached to the substrate and interconnected with conductive paths or “traces” on the substrate. The traces are typically created on the surface of the substrate by coating the entire surface of the substrate with a layer of copper, masking the copper with the interconnect pattern using a photolithographic process, and selectively etching away the non-masked copper. The minimum separation of the traces is often limited by the etching process. More complicated circuits are fabricated using multiple layers of circuit traces separated by insulating layers with connections between the conductive layers formed by holes between the insulating layers that are filled with conductive material. These interlayer connections are called “vias.” A rigid substrate with one or more layers of circuit traces is referred to as a printed circuit board (PCB) and an electronic assembly that is formed by mounting electronic components to the PCB is a printed circuit board assembly (PCBA).
The drive to fit electronics into smaller or curved packages drove the development of flexible substrates where the traces are created by plating and etching as done with the rigid substrates or screen printing a conductive material onto the flexible substrate. These flexible printed circuits (FPCs) are limited in the separation of circuit elements, referred to as the “pitch” of the traces, in the same way as convention rigid PCB fabrication as they use the same processes to form the circuits.
The ability to directly print circuit elements has been developed in the last decade or so to take advantage of low-cost printing technologies. Common printing processes such as screen printing, gravure, offset lithography, and inkjet have been used to create circuits using both conductive carbon-based compounds and metals. Each of the processes have advantages and disadvantages related to resolution, throughput, and cost. Circuits fabricated from carbon-based compounds have a lower conductivity than metal circuits. The metal inks require temperatures of up to 300° C. to fuse the metal particles into a continuous conductive strip, limiting the substrate to materials that are stable at this temperature.
It would be beneficial to be able to produce highly conductive circuits and circuit elements on both rigid and flexible substrates with a finer pitch than possible with current printing technologies and/or without requiring elevated process temperatures.